1. Field of the Invention
The present invention relates to a light emitting panel in which a light emitting element formed on a substrate is enclosed between the substrate and a cover member. Also, the present invention relates to a light emitting module in which an IC or the like is mounted on the light emitting panel. Note that, in this specification, the light emitting panel and the light emitting module are generically called light emitting devices. The present invention further relates to a method of driving the light emitting device and an electronic appliance using the light emitting device.
2. Description of the Related Art
A light-emitting element emits light by itself, and thus, has high visibility. The light-emitting element does not need a backlight necessary for a liquid crystal display device (LCD), which is suitable for a reduction of a light-emitting device in thickness. Also, the light-emitting element has no limitation on a viewing angle. Therefore, the light-emitting device using the light-emitting element has recently been attracting attention as a display device that substitutes for a CRT or the LCD.
Incidentally, the light-emitting element means an element of which a luminance is controlled by electric current or voltage in this specification. The light emitting element includes an OLED (organic light emitting diode), an MIN type electron source element (electron emitting elements) used to a FED (field emission display) and the like.
The OLED includes a layer containing an organic compound in which luminescence generated by application of an electric field (electroluminescence) is obtained (organic light emitting material) (hereinafter, referred to as organic light emitting layer), an anode layer and a cathode layer. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) exist as the luminescence in the organic compound. The light-emitting device of the present invention may use one or both of the above described light emissions.
Note that, in this specification, all the layers provided between an anode and a cathode of the OLED are defined as the organic light emitting layers. The organic light emitting layers specifically include a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, an electron transporting layer and the like. These layers may have an inorganic compound therein. The OLED basically has a structure in which an anode, a light emitting layer, a cathode are laminated in order. Besides this structure, the OLED may take a structure in which an anode, a hole injecting layer, a light emitting layer, a cathode are laminated in order or a structure in which an anode, a hole injecting layer, a light emitting layer, an electron transporting layer, a cathode are laminated in order.
FIG. 41 shows a configuration of a pixel of an ordinary light emitting device. The pixel shown in FIG. 41 has TFTs 50 and 51, a storage capacitor 52 and a light emitting element 53.
In the TFT 50, a gate is connected to a scan line 55, one of a source and a drain is connected to a signal line 54, and another one thereof is connected to a gate of the TFT 51. In the TFT 51, a source is connected to a power supply 56 and a drain is connected to an anode of the light emitting element 53. A cathode of the light emitting element 53 is connected to a power supply 57. A storage capacitor 52 is provided for holding a voltage between the gate and the source of the TFT 51.
When a voltage of the scan line 55 turns the TFT 50 ON, a video signal input to the signal line 54 is input to the gate of the TFT 51. When the video signal is input, a gate voltage (a voltage difference between the gate and the source) of the TFT 51 is determined in accordance with the voltage of the input video signal. A drain current of the TFT 51, which flows via the gate voltage, is supplied to the light emitting element 53, and the light emitting element 53 emits light via the supplied current.
Since a TFT formed with poly-silicone is higher than a TFT formed with amorphous silicon in the field-effect mobility and has a large ON-current, the TFT formed with poly-silicone is more suitable to a transistor for a luminescent panel than the TFT formed with amorphous silicon.
However, the electrical characteristics of the TFT utilizing the poly-silicon can not compared to the characteristics of a MOS transistor formed on a, what is called, single crystal silicon substrate. For example, the field-effect mobility of the TFT utilizing the poly-silicone is 1/10 or less of the single crystal silicon. Further, the TFT utilizing the poly-silicon has such a defect as that dispersion of the characteristics is caused easily from defects formed in the boundary of crystal grains.
In the pixel shown in FIG. 41, when the characteristics such as threshold, ON-current or the like of the TFT 51 disperse at each pixel, even when the voltage of the video signal is the same, the magnitude of the drain current of the TFT 51 varies among the pixels resulting in the dispersion of the luminance of the light emitting element 53.
A problem, which resides in putting a light emitting device utilizing an OLED into practical use, is the short life of the OLED due to a deterioration of the organic luminescent layer. The organic luminescent material is weak to moisture content, oxygen, light and heat; and the deterioration thereof is accelerated thereby. Particularly, speed of the deterioration depends on the configuration of a device that drives the light emitting device, the characteristics of the organic luminescent material, the material of the electrode and the driving method of the light emitting device, or the like.
Even when the voltage impressed the organic luminescent layer is the same, when the organic luminescent layer deteriorates, the luminance of the OLED is decreased resulting in an unclearness of the displayed picture.
Further, the temperature of the organic luminescent layer depends on the temperature of outside air or the heat emitted from the OLED panel itself or the like. Generally, in the OLED, the value of the current flow varies depending on the temperature. Particularly, when the voltage is the same, when the temperature of the organic luminescent layer increases, the current that flows to the OLED becomes larger. And since the current that flows to the OLED and the luminance of the OLED are in a proportional relationship, the larger current that flows to the OLED leads to the higher luminance of the OLED. As described above, since the luminance of the OLED varies depending on the temperature of the organic luminescent layer, it is difficult to display a desired tone, and accompanying to a raise of the temperature, the current consumption of the light emitting device becomes larger.